- Radial keratotomy (RK) is a type of cornea surgery developed in the Soviet Union by Dr. Syvatoslav Fyodorov. The surgery is designed to improve myopia (nearsightedness). The ultimate goal is to restore the patient’s vision to 20/20 without the use of contact lenses or eyeglasses. The words radial keratotomy refer to the radial cuts or incisions made on the cornea that form a spoke-like pattern. Those with mild myopia can expect the best results. Possible candidates must require glasses or contact lenses to correct their vision. The best results are seen in those with a refraction between -2.00 to -4.00 diopters and a visual acuity level of 20/80 to 20/200. Those with high myopia (-5.00 or greater) will not achieve the same results. Although prospective RK patients must be mildly myopic, people with very slight myopia are discouraged from undergoing surgery for such a minimal correction. Those with corneal disorders, glaucoma or pre-glaucomatous conditions, lenticular astigmatism, or other eye disorders are also often not considered good candidates for this surgery. The presurgical examination involves an external exam of the eye, a vision acuity test, and a refraction test. Additional tests may include corneascope photographs, a slit-lamp examination, intraocular-pressure testing, motility studies, and cornea measurement and depth testing. Approximately one hour before surgery, the patient is treated with two or more sets of eye drops to dilate and anesthetize the eye. Many surgeons also administer an antibiotic. In the operating room, the surgeon takes a number of exact readings and measurements of the eye to determine the surgical plan. The cornea is stamped with a trephine, a cookie-cutter-like instrument that marks an incision pattern that the surgeon uses as a guide. The pattern includes a center circular clear zone and a variable number of incision lines radiating out from the clear zone. The surgeon uses a diamond blade to cut from the clear zone circle outward. The procedure flattens or shortens the cornea, to produce better refraction and improved vision. The surgery is done on an outpatient basis and generally requires thirty minutes per eye. After the surgery, the eye is covered with a patch for twenty-four hours, and eye drops are used for a week. There may be some pain, sensitivity to light, tearing, a decrease or fluctuation of visual acuity, over- or under-correction, and astigmatism. Patients can achieve from 20/20 vision or no improvement at all, although studies indicate that radial keratotomy improves vision in the majority of cases. However, the procedure remains a controversial one and prospective candidates are encouraged to research the procedure, its risks, and benefits, as well as the surgeon involved.
Refraction assessment determines the normal or abnormal bending of the lightwaves as they pass through the cornea and lens. Conditions have to be just right for the light to focus properly on the retina. A refraction assessment will help the doctor determine which corrective lens prescription is needed to give the sharpest vision. A computerized refractor is used to measure the eyes or the doctor may use a technique called retinoscopy. In this procedure, the doctor shines a light into the eye and measures the refractive error by evaluating the movement of light that is reflected by the retina. The refraction assessment is fine-tuned with the use of a phoroptor, a masklike device that contains wheels of different lenses. The Snellen chart is again viewed through these lenses with the best combination judged for the sharpest vision. By repeating this step several times, the doctor will be able to find the lens that will give the greatest possible acuity.
Refractive error (See Ametropia.)
Retina is a thin layer of tissue on the inside back wall of the eyeball between the choroid and the vitreous gel. The term comes from a Latin word meaning “net”, aptly named because the retina consists of millions of light-sensitive cells and nerve cells that capture images focused onto them by the cornea and lens. After the light gets through the vitreous body, the image has traveled approximately an inch through the eye to the target, the retina. The light-sensitive cells, sometimes called photoreceptors, are either rods or cones. Rods allow sight in a very dim light or off to the side while looking ahead (peripheral or side vision), but they cannot distinguish colors. Cone cells distinguish colors very well but require more light to function. Cone cells are concentrated in the center of the retina. Light striking the rods and cones trigger a chemical reaction which, in turn, generates electrical impulses that are relayed through the optic nerve to the visual cortex, the seeing portion of the brain. The image received by the retina is upside-down and reversed as a result of the convex shape of both the cornea and the lens. The brain reinterprets the information allowing sight in a correct orientation. The brain must also merge the image of both eyes to produce a clear picture. The outer part of the retina receives its nutrition from retinal blood vessels. The rods outnumber the cones by an average of twenty to one and are scattered throughout the retina. In all, there are over 130 million light sensitive cells in the retina of each eye: about 125 million rods and about six and one half million cones. Because the rods are scattered throughout the retina, unlike the cones which are concentrated in the center, they are responsible for peripheral vision. Since the rods do not require high levels of light to function, they enable the eye to see at night. Most of the cones are concentrated in the macula. This foveal cramming is 147,000 per square millimeter, which is one of the reasons this spot has the keenest sight. It can be compared to a good laser printer at 5,500 dots per square millimeter. The reason the retina has so many cells is because millions of bits of information must be gleaned from a typical scene in a fraction of a second. Some cell populations are organized to distinguish only very distinct patterns. For instance, some will only signal a black vertical line on a light background, others a horizontal line, and still others for oblique angles. There are some groups which signal borders, but only borders in a particular orientation. Other arrays of cells respond to motion, but only one explicit direction of motion. While all this is going on, there have to be cells which respond to colors, shapes, and forms. And all this happens in just a split second. Approximately one million nerve fibers carry electrical impulses generated by retinal cells leave the eye via the optic nerve at an estimated speed of about one billion bits per second.
Retinal edema is a condition in which the capillaries of the retina bleed, filling the spaces between the retinal cells with fluid. Retinal edema may be present throughout the retina, or it may be contained in a specific area. If the macula is spared, vision may be relatively unaffected at first. However, the macula tends to accumulate fluid, causing macula edema. The condition is characterized by inflammation of the area and blurred or impaired vision. If untreated macular edema may develop into cystoid degeneration, and central vision may be permanently lost. Retinal edema is caused mainly by diabetes, hypertension, retinal vein obstruction, traction of the vitreous, such inflammations as uveitis and retinitis, and cataract surgery. Retinal edema is diagnosed with fluorescein angiography and may resolve itself spontaneously or persist. Persistent cases may cause permanent vision loss. Treatment varies according to the cause.
Retinal examination involves the use of dilating drops to open the pupils as wide as possible, providing a bigger window to the back of the eye. Using a slit-lamp or ophthalmoscope, the doctor can diagnose abnormalities in the vitreous, retina, optic nerve, and choroid. These areas hold important clues for the presence of such disease elsewhere in the body as diabetes or high blood pressure. Some doctors use another instrument called an indirect ophthalmoscope, which uses a bright light mounted on the doctor’s head, something like a miner’s lamp. This allows more detail to be seen inside the eye and in three dimensions. Because the effects of the dilating drops usually lasts for several hours, it is always advised that someone else drive the patient home and that he/she wears sunglasses.
Retinoblastoma is a malignant eye tumor that originates in the retina. It is the most common childhood ocular tumor and may be the most common congenital tumor of any kind. Retinoblastoma may be either hereditary or sporadic. As many as 40% of the cases are hereditary and is usually present at birth and most frequently appears at approximately one year of age, often affecting both eyes. Children of families with histories of retinoblastoma should be examined regularly for tumors. Even without prior family history, all bilateral retinoblastoma tumors should be considered hereditary and capable of being passed to future generations. The more common sporadic type is not hereditary and usually affects only one eye, often appearing at approximately two years of age. All forms of the disease are evident by age five in almost all cases. Symptoms include redness, pain, and inflammation in the early stages. When the tumor has grown larger, the eyes may cross or the pupil may change from black to white or gray. This change in pupil color, called leukokoria, is the actual appearance of the tumor itself, visible through the hole of the pupil. After the tumor is diagnosed, treatment must be given immediately. Both types of retinoblastoma are life threatening and can grow and spread rapidly up the optic nerve to the brain. Sometimes the affected eye has to be enucleated, or removed. In the case of bilateral retinoblastoma, the more seriously affected eye may be enucleated and the less affected eye given radiation or chemotherapy treatments. Cause is the absence of paired retinoblastoma genes on the 13th chromosome. This can occur from hereditary and/or environmental causes. Although the condition has one of the highest rates of spontaneous regression among all tumors, the cause of regression is unknown.
Retinopathy of prematurity (ROP) is a condition that appears soon after birth, generally in premature infants, in which abnormal blood vessels develop in the retina. This condition may progress to a retinal detachment that may cause blindness. Although the precise cause is unknown, it is often seen in premature infants, those with very low birth weight (less than 1,000 grams) as the major factor. These premature infants may develop an overabundance of blood vessels in the retina. This may resolve spontaneously or may result in hemorrhages and the development of fibrous tissue or scar tissue. High myopia, uveitis, glaucoma, amblyopia, strabismus, and retinal detachment may follow as secondary conditions. Total blindness may result. In approximately 80% of ROP cases, the retinal vessels heal themselves within the first year of the child’s life. About 15% of the remainder of cases progress to mild or modest forms of ROP. The remaining 5% of the cases develop severe ROP with retinal detachment and other secondary conditions. Treatment has recently included cryosurgery, or freezing therapy, which has shown to have good results and, if the ROP is severe, has proven to be the most effective treatment. If retinal detachment occurs, it is difficult to treat, resulting in only 30%-50% success rates. Researchers at the Department of Newborn Medicine at the Royal Alexandra Hospital in Edmonton, Alberta, have found that a dose of vitamin E given within 12 hours of birth may reduce the incidence of severe forms of retinopathy of prematurity. Historically, ROP was called “retrolental fibroplasia” and thought to be caused by an overabundance of oxygen used in the incubators of premature infants. Modern, well-managed neonatal intensive care units for premature infants monitor oxygen, electrolytes, vitamins, and nutrition, carefully maintaining each at the best level for normal growth and development. Despite these efforts, ROP may be unavoidable for very low birth weight premature infants.
Retinoscope is an instrument used during an eye exam to determine the refractive power of the eye. The examiner shines the light of the retinoscope into the eye from an arm’s distance away. As the light moves across the pupil, the speed and direction of the lights and shadows are evaluated and recorded. This data indicates to the examiner the refractive power of the eye and is used to determine nearsightedness, farsightedness, or astigmatism.
Retrobulbar neuritis is a form of optic neuritis, or swelling of the optic nerve, associated with multiple sclerosis. The condition is commonly seen among younger patients with MS and is often a first symptom of the disease. Symptoms of retrobulbar neuritis include sudden loss of vision in one eye, accompanied by pain associated with eye movement. Blind spots, or scotomas, may appear within the field of vision, as well as color blindness and difficulty seeing in bright light. Vision may be affected in the entire field or may begin in the center of the field and progress to peripheral fields. Retrobulbar neuritis also can be caused by a neurological lesion, and is not necessarily a sign of MS. The condition may be treated with the drug ACTH and with oral steroids. Although recurrence is possible, the prognosis for remission is good and patients usually recover partial or full vision within three months.